A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Typical trace lengths are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. Conventionally each trace records a source activation so there is one trace for each live source location-receiver activation. Note that, for purposes of the instant disclosure, the term “source” should be understood to mean either a single seismic signal device or a collection of seismic signal devices (which might be clustered or spaced apart) that are to be activated simultaneously or both. It should be clear from the context in which is intended. In a typical 2-D survey, there will usually be many source activation at different locations which can result in several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces.
In seismic acquisition a marine source array, usually an array of air guns, is composed of many single units that are towed behind a vessel that travels over the survey area. These units (e.g., air guns, water guns, sparkers, boomers, chip systems, water sirens, etc.) are typically hung in a line under a sausage buoy to allow them to be towed in a streamlined fashion. It is typical in deep water seismic surveying to use 6 to 8 guns under a single buoy. This configuration of seismic sources is conventionally referred to as a sub-array. For purposes of the instant disclosure, the term “array” will be understood to refer to the totality of sources that are to be utilized in a seismic survey, in the marine case whether towed by one boat or two or more. A subarray (which might consist of one or more sources) will be understood to be a subset of the sources that are to be activated simultaneously.
If more guns are needed to achieve the desired signal properties, additional subarrays are typically used to prevent the sausage buoy and seismic gun array from becoming too long to fit in the available space on deck and to prevent it from becoming excessively directional and excessively long. Excessively long and directional arrays have undesirable properties. For example, if the number of guns necessitates an array that is several tens of meters long, not only would the buoy and array be too long to fit onto a typical seismic vessel, but the signal emitted by the array would be received by the seismic receivers with a great deal of differential normal moveout. That is, because of the different locations within the subarray the signal path and travel time for each source varies enough such that the difference amounts to a fair fraction of a temporal period. Thus, the higher frequency components will tend to be out of phase and thereby attenuating.
Heretofore, as is well known in the seismic acquisition and processing arts, there has been a need for a system and method that provides a more efficient method of acquiring and processing seismic data that does not suffer from the disadvantages of the prior art. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present embodiments, however, it should be noted and remembered that the description which follows, together with the accompanying drawings, should not be construed as limiting the claims to the examples (or embodiments) shown and described. This is so because those skilled in the art will be able to devise other forms of this disclosure within the ambit of the appended claims.